In many systems, such as within many electronic circuits, impedance mismatches can cause an undesired amount of return loss, which may result for instance from some amount of signal reflection. In metallic conductor systems, for example, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. Signal reflection may occur when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, where some of the signal power is reflected back to its origin rather than being carried all the way along the cable to the far end. This signal reflection may occur due to impedance mismatches along the transmission path.
In radio frequency (RF) design practice, this is often measured in a dimensionless ratio known as VSWR. The ratio of energy bounced back depends on the impedance mismatch. In RF systems, an attenuator may be implemented to ensure that, in addition to acting as an attenuator, the RF impedance (return loss) of both the input and output of the attenuator is held as close as possible to the desired system impedance. Failure to maintain a proper impedance match can greatly affect the system frequency response (power transfer) and noise figure.
When receiving a broadband of signals with multiple receivers, maintaining return loss on a single input pin is particularly difficult. Return loss tends to degrade significantly as the number of parallel paths connected to the single input is increased. In some systems, multiple, parallel input paths may exist with signals coming from one input source. It is often desirable to maintain a relatively constant return loss for the system. Thus, the system may be implemented with a fixed impedance between its input and output, thereby providing a constant return loss through the system. For instance, the impedance of each of the multiple paths may be fixed to provide a desired impedance match with the input source.